1. Field of the Invention
The present invention relates to a switching device and more particularly to a reusable, fast opening switch for producing electrical power pulses.
2. Background of the Invention
Electrical power pulses have been produced by charging an electrostatic energy store, such as a capacitor, and then discharging it, by means of a closing switch, into a load. Because known electrostatic energy stores have a very low energy density, they are generally of large physical dimensions.
Electromagnetic energy storage systems, having very high energy densities are also known. However, when used in connection with short pulse duration (&lt;&lt;1 microsecond), high power, pulse forming networks prior art closing switch technology limits the power gain per stage of the electromagnetic energy storage systems to about a factor of 3. This is because prior art closing switch technology is basically directed to fuse type devices, which, as they get larger, require so much of the energy in the storage device to "switch" it, that very little power gain can be realized at the load.
For a typical pulse forming switching system, the power to the switch, P.sub.in, is equal to the energy supplied from the source (W.sub.o) divided by the time it takes to charge up the storage device (t.sub.o). ##EQU1##
The power delivered to the load is equal to the efficiency of the switching multiplied by the energy supplied (W.sub.o) divided by the time it takes to deliver the energy from the storage device to the load (t.sub.i). ##EQU2## Therefore the power gain, A, which is the ratio of P.sub.out over P.sub.in, can be described as follows: ##EQU3## When the power gain of such systems is about 3, the efficiency drops off as t.sub.o goes up. Therefore, for high power requirements many large stages are required. A fast opening switch would permit the use of magnetic energy storage which may typically have up to one thousand times the energy density of an electrostatic system. However, for such an energy storage system a switch having a large impedance change from a low resistance to a high resistance resulting in a correspondingly large power gain is required.
Moreover, fused wire type switches generally have a relatively low breakdown electric field (about 20 kilovolts per centimeter) and have to be replaced after each "shot". Since many "fuses" are usually involved in such switching systems, there are significant economic and technical disadvantages to the fuse switch approach. In addition, most fuse switches have switching times well in excess of 10.sup.-8 seconds and, as alluded to above, are relatively inefficient, absorbing about 30 percent of the energy available in the energy storage inductor.
Therefore, a need exists for a fast opening (on the order of 1 nanosecond) switch, capable of switching between a highly conductive and highly resistive state and able to handle large current densities. To be useful for pulsed power source application, the switch must be able to switch back and forth repeatedly without damage.
Vance, in U.S. Pat. No. 3,399,330, discloses a solid state switch which changes from a high resistivity to a low resistivity as a current pulse is applied. Return to high resistivity is accomplished by reversing the polarity of current flow. While such a device may be useful for computer memories or the like, it is not applicable to a pulse power generator application where a shift from low resistivity to high resistivity is required in order to produce the desired high power, short duration pulse.
Geishecker, in U.S. Pat. No. 3,955,170, discloses a thermally activated opening switch. In accordance with the device of Geishecker, as the temperature of the device increases, the resistivity of an associated semiconductor also increases. Thus, the device of Geischecker displays a positive temperature coefficient (PTC). Geishecker is representative of many PTC devices presently available. In a 1967 article in Ceramic Age (Vol. 83, pages 44-47, May 1967) there are described many such devices. These PTC devices are used as switches to perform control, detection and regulatory functions.
Barium titanate, the semiconductor material mentioned by Geishecker at column 1, line 42, is not metallic in its lower resistivity state and its low resistance value is relatively high as compared to metallic conductor materials, about 30 ohm-cm. Such a high impedance renders the material unsuitable for use in a pulse power application. While there is no hard barrier in terms of initial resistivity, for practical and efficient operation the initial resistivity of a switch for high power applications should be less than about 10.sup.-2 ohm-cm. As will be further developed below, an additional disadvantage attendant to high initial resistivity is that a large shock wave, proportional to the resistance, is created during switching. If high current densities are to be switched, the resistance must be kept low to prevent the creation of a shock wave which will be larger than the yield strength of the switch material.
In addition, pulse power applications require that a very large increase in resistivity take place at the switch transition from low to high resistivity. In order to be effective the ratio of high to low resistivity must be at least a factor of 400. Higher ratios will yield cleaner switching with diminishing returns as the ratio increases. While in barium titanate, the high/low resistivity ratio is a factor of about 10.sup.4, the high initial resistivity and slow switching time make this material impractical for high power pulse generation.
Moreover, the PTC-semiconductor transitions in material such as barium titanate involve changes of crystal symmetry. These structural changes seriously impair the usefulness of such materials when large current densities are involved since such materials tend to crack when subjected to large current densities due to the large and rapid changes in crystal structure which occur during transitions.